The present invention relates to an automatic diaphragm control device of the objective used in a CCTV camera which has recently found many applications.
Diaphragm control of a television camera objective utilizing a video signal coming from the television camera can be achieved sometimes with a control signal derived from said video signal which has directly been rectified, but this is not always possible, since a difference in the absolute brightness of an object to be picked up would be reflectd by a difference in the corresponding video signal level and such video signal level depends also upon the type of camera. In this situation, the optimum picture will be obtained only when the video signal coming from the television camera is appropriately controlled to form a desired diaphragm control signal. The ratio between the bright portion and the dark portion or the contrast value of the object reaches, in the natural world, for example, a level as high as tens : one even under the cloudy sky and often reaches hundreds : one under the blue sky. In the television-reproduced picture, on the contrary, the contrast value is limited to an order of 30 through 40:1. Accordingly, to obtain the optimum picture in the natural world, a proportion of the video signal corresponding to the bright portion in the whole picture must be appropriately controlled to produce a desired diaphragm control signal. Moreover, in view of a fact that various types of CCTV camera have recently been adopted in various fields for many purposes, the diaphragm control of the objectives incorporated in these cameras can not be achieved with high stability unless said diaphragm control signal can be obtained with a wide range of source voltages with high stability.